The nitride semiconductor system that includes AlxInyGa1-x-yN is a desirable direct-bandgap semiconductor material system for light-emitting devices operating in the visible and green-blue-ultraviolet spectrum. However, nitride semiconductors are difficult and costly to produce as bulk single crystals. Therefore, hetero-epitaxial technology is often employed to grow nitride semiconductors on different material substrates such as sapphire or SiC by metal-organic chemical vapor deposition (MOCVD) or other epitaxial growth techniques, including, but not limited to hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE) and liquid phase epitaxy (LPE). In order to improve the crystalline quality of the grown layers, buffer layer growth at low temperature, patterning, epitaxial lateral overgrowth, or additional growth steps may be required to reduce crystal defects to levels necessary for operation of light-emitting devices. Further improvements in crystalline quality are needed to enable development of smaller light-emitting devices with longer life time, higher output power, and lower cost relative to conventional devices.
Presently, nitride semiconductor structures grown on sapphire substrates are used for conventional blue LED, green LED, ultraviolet (UV) LED, and blue LD devices. These devices have applications in a variety of devices including full-color displays, traffic lights, image scanners, solid state lighting and high-density optical storage disks.
Because sapphire has a low thermal conductivity and is electrically insulating, the functionality of nitride semiconductor structures on sapphire is limited. Both electrical contacts of the light-emitting device grown on a sapphire substrate have to be located on the top surface to form a lateral type device. This reduces the usable area of light-emission when compared to a GaN light-emitting device formed on conductive (i.e., highly doped semiconductor) substrates that require only one contact on the top surface and another contact on the substrate (i.e., a vertical type device). Because both contacts are located on the top surface in a lateral device, significant lateral current flows through the chip resulting in heating of the light-emitting device which accelerates the degradation of the device. Device manufacturers have attempted to overcome these challenges by removing the devices from the sapphire substrate following growth using techniques such as laser lift-off and physical and chemical removal of the sapphire substrate. However, these approaches present many problems, including high capital costs, resultant damage to the device layer, and low yields. The coefficient of thermal expansion of sapphire is also poorly matched to gallium nitride and its alloys. As a result, the growth of gallium nitride-based films on sapphire substrates presents challenges that scale with wafer diameter. Because of these challenges, manufacturers have found it difficult to move to larger substrate sizes despite the potential for associated cost reductions. The CTE related challenges are not addressed by post-device growth sapphire substrate removal techniques.
Recently, interest has grown in LEDs capable of emitting in the UV region (wavelength <400 nm). For LED devices emitting at wavelengths shorter than the bandgap of GaN at ˜365 nm, the thick buffer layer of GaN used in conventional growth on sapphire substrate reduces the useful light output by approximately half due to absorption of light emitted from the AlxInyGa1-x-yN active region by the narrower bandgap GaN.
Recently, researchers have made progress in the growth of III-nitride based devices, including LDs and LEDs, on freestanding GaN manufactured by HVPE. Because of the low dislocation material that is possible in freestanding GaN, devices grown on high quality freestanding GaN have demonstrated significant performance improvements over those grown on sapphire or silicon carbide as presented by T. Nishida, et. al. in “Highly efficient AlGaN-based UV-LEDs and their application as visible light sources,” Proceedings of SPIE Vol. 4641 (2002), by H. Hirayama, et. al. in “High-efficiency 352 nm quaternary InAlGaN-based ultraviolet light-emitting diodes grown on GaN substrates,” Japanese Journal of Applied Physics, Vol. 43, No. 10A, 2004, or by D. W. Merfeld, et. al. in “Influence of GaN material characteristics on device performance for blue and ultraviolet light-emitting diodes,” Journal of Electronic Materials, Vol. 33, No. 11, 2004. However, for this approach to be commercially viable, it is necessary to reduce the cost of the freestanding GaN material used in the devices. For LEDs, it is also necessary to develop techniques for reducing the thickness of the conductive GaN substrate within the final device structure to reduce free-carrier absorption in the substrate and unwanted emission from the sides of the substrate. At present, thinning of freestanding GaN substrates in the finished device structure is not viable due to the very high cost of the freestanding GaN substrate and the difficulty of controllably and selectively removing the thick (typically >200 μm thick) GaN substrate without damaging the thin device structure (typically <5 μm thick).